FIG. 1 illustrates an exemplary communication network 100, which includes a number of user terminals 102 competing for access to a gateway terminal 106 via a communication satellite 104. A number of access methods have been developed to permit multiple user terminals 102 to access gateway terminal 106. Fixed Frequency Division Multiple Access (FDMA) is an access method in which each user terminal 102 is assigned a respective fixed frequency and may transmit simultaneously with one or more other user terminals 102, each terminal transmitting on a different frequency. Time Division Multiple Access (TDMA) is an access method in which multiple user terminals 102 are assigned respective time slots at a same frequency and may transmit in rapid succession using their respective time slots, each terminal using a different time slot. However, in FDMA and TDMA, respectively, the user terminals may use the assigned frequency or the assigned time slots intermittently. Therefore FDMA and TDMA access methods are inefficient.
In the early 1970s, a random access method called Aloha was introduced. Aloha allows each user terminal 102 to transmit at will in a same frequency. When multiple user terminals 102 transmit simultaneously, their respective transmissions become corrupted, each multiple user terminal 102 selects a random, and most probably different respective delay, and retransmits after expiration of the respective delay.
Sometime after the introduction of Aloha, a revised access method known as Slotted Aloha, or S-Aloha was introduced. Using S-Aloha, user terminals 102 begin their transmission on a common time marker and have a same transmission duration. S-Aloha reduced, with respect to Aloha, a probability of two user terminals 102 transmitting simultaneously by about 50% due to elimination of partially overlapping transmissions by separate terminals. It is well known that maximum channel utilization for S-Aloha is e−1, or about 37%, whereas maximum channel utilization for Aloha is 0.5 e−1, or 18.5%. However, both Aloha and S-Aloha become unstable when operating close to capacity because retransmission of previously unsuccessful transmissions tie up the channel. To keep the channel stable, actual channel utilization is kept much lower than the maximum channel utilization stated above.
To improve throughput and minimize delay, it became clear that a user terminal could transmit multiple copies of information in different time slots and have a receiving device sort out duplicate information and request retransmission only when none of the multiple copies of the information are correctly received. Devices in most modern wireless communication systems establish initial communications in this way, which is known as Diversity S-Aloha. In systems using Diversity S-Aloha, typically two or three copies of the information are transmitted.
In the early 2000's, it was observed that, if a receiving device knows locations of duplicated transmissions within received information, and one of the duplicated transmissions is received without corruption, then the receiving device may use the one of the duplicated transmissions to cancel another copy of the duplicated transmission that corrupts a received transmission from a second user terminal, thereby increasing a probability of clear reception. If cancellation is performed iteratively, a probability of receiving uncorrupted transmissions is increased and channel capacity is improved beyond that of Diversity S-Aloha. Such a technique is known as Contention Resolution Diversity Slotted Aloha (CRDSA).
Also in the early 2000's, an access scheme was developed based on low-rate forward error correction (FEC) coding and scrambling codes with iterative interference cancellation performed at a receiving device such that a large number of transmissions, within a time slot, from different terminals can be correctly separated and decoded. The access scheme is known as Scrambled Code Multiple Access (SCMA) and provides much greater capacity than S-Aloha, Diversity S-Aloha, and CRDSA.